Typically, semiconductor chips are tested to verify that they function appropriately and reliably. This is often done when the semiconductor chips are still in wafer form, that is, before they are diced from the wafer and packaged. This allows the simultaneous testing of many semiconductor chips at one time, creating considerable advantages in cost and process time compared to testing individual chips once they are packaged. If chips are found to be defective, they may be discarded when the chips are diced from the wafer, and only the reliable chips are packaged.
Generally, modern microfabricated (termed MEMS) probe card assemblies for testing semiconductors have at least three components: a printed circuit board (PCB), a substrate to which thousands of probe contactors are coupled (this substrate hereinafter will be referred to as the “probe contactor substrate”), and a connector (also referred to herein as an interposer). The interposer electrically interconnects the individual electrical contacts of the PCB to the corresponding electrical contacts on the probe contactor substrate which relay signals to the individual probe contactors. In most applications the PCB and the probe contactor substrate must be roughly parallel and in close proximity, and the required number of interconnects may be in the thousands or tens of thousands. The vertical space between the PCB and the probe contactor substrate is generally constrained to a few millimeters by the customary design of the probe card assembly and the associated semiconductor test equipment. Conventional means of electrically connecting the probe contactor substrate to the contact pads of the PCB include solder connection, elastomeric vertical interposers, and vertical spring interposers. However, these technologies have significant drawbacks.
In the early days of semiconductor technology, the electrical connection between the probe contactor substrate and the PCB was achieved by solder connection. Solder connection technology involves electrically connecting an interposer to the PCB by means of melting solder balls. For instance, U.S. Pat. No. 3,806,801, assigned to IBM, describes a vertical buckling beam probe card with an interposer situated between the probe contactor substrate and a PCB. The interposer is electrically connected to the PCB, terminal to terminal, by means of melting solder balls (see FIG. 5). Another example is seen in U.S. Pat. No. 5,534,784, assigned to Motorola, which describes another probe card assembly with an interposer that is solder reflow attached to a PCB by using an area array of solder balls. The opposite side of the interposer is contacted by buckling beam probes (see FIG. 6).
In both of these patents, an array of individual probe contactor springs is assembled to the interposer, either mechanically or by solder attachment, which use solder area array technology. However, this method has a number of significant disadvantages, particularly when applied to large area or high pin count probe cards. For instance, probe cards with substrate sizes larger than two square inches are difficult to solder attach effectively because both the area array interconnect yield and reliability become problematic. During solder reflow, the relative difference in thermal expansion coefficients between the probe contactor substrate and PCB can shear solder joints and/or cause mismatch-related distortion of the assembly. Also, the large number of interconnects required for probe cards makes the yield issues unacceptable. Furthermore, it is highly desirable that a probe card assembly can be disassembled for rework and repair. Such large scale area array solder joints can not be effectively disassembled or repaired.
An alternative to solder area array interposers is the general category of vertically compliant interposers. These interposers provide an array of vertical springs with a degree of vertical compliance, such that a vertical displacement of a contact or array of contacts results in some vertical reaction force.
An elastomeric vertical interposer is an example of one type of a vertically compliant interposer. Elastomeric vertical interposers use either an anisotropically conductive elastomer or conductive metal leads embedded into an elastomeric carrier to electrically interconnect the probe contactor substrate to the PCB. Examples of elastomeric vertical interposers are described in U.S. Pat. No. 5,635,846, assigned to IBM (see FIG. 7), and U.S. Pat. No. 5,828,226, assigned to Cerprobe Corporation (see FIG. 8).
Elastomeric vertical interposers have significant drawbacks as well. Elastomeric vertical interposers often create distortion of the probe contactor substrate due to the forces applied on the probe contactor substrate as a result of the vertical interposer itself. These forces are necessary to ensure that the probe contactor substrate maintains a reliable electrical contact to the PCB. Additionally, elastomers as a material group tend to exhibit compression-set effects (the elastomer permanently deforms over time with applied pressure) which can result in degradation of electrical contact over time. The compression-set effect is accelerated by exposure to elevated temperatures as is commonly encountered in semiconductor probe test environments where high temperature tests are carried out between 75° C. and 150° C. or above.
A second type of vertical compliant interposer is the vertical spring interposer. In a vertical spring interposer, springable contacting elements with contact points or surfaces at their extreme ends, extend above and below the interposer substrate and contact the corresponding contact pads on the PCB and the probe contactor substrate with a vertical force. Examples of such vertical spring interposers are described in U.S. U.S. Pat. No. 5,437,556, assigned to Framatome (see FIG. 9A), U.S. Pat. No. 5,800,184, assigned to IBM (see FIG. 9B), U.S. Pat. No. 6,624,648 assigned to FormFactor (see FIG. 10A) and U.S. Pat. No. 6,624,648 assigned to FormFactor (see FIG. 10B).
However, vertical spring interposers (and also elastomer type vertical interposers) have significant disadvantages. In order to achieve electrical contact between the PCB and the substrate with probe contactors, the interposer springs must be compressed vertically. The compressive force required for a typical spring interposer interconnect is in the range of 1 gf to 20 gf per electrical contact. The aggregate force from the multitude of vertical contacts in the interposer causes the probe contactor substrate to bow or tent since it can only be supported from the edges (or from the edges and a limited number of points in the central area) due to the required active area for placement of probe contactors on the substrate. The tenting effect causes a planarity error at the tips of the probe contactors located on the surface of the probe contactor substrate (see FIG. 11).
This planarity error resulting from vertical interposer compression forces requires that the probe contactor springs provide a larger compliant range to accommodate full contact between both the highest and the lowest contactor and the semiconductor wafer under test. The increase in compliant range of a spring, where such increase is roughly equal to the planarity error, requires that the spring be larger, with all other factors such as contact force and spring material being constant, and hence creates a deleterious effect on probe pitch. Furthermore, probe contactor scrub is often related to the degree of compression, so the central contactors in the tented substrate will have different scrub than the outer contactors which are compressed less. Consistent scrub across all contactors is a desirable characteristic, which is difficult to achieve with vertical compliant interposers.
A novel and innovative approach to interposers is U.S. patent application Ser. No. 11/226,568 (the '568 application) by Raffi Garabedian (a common inventor to the present application), Nim Tea and Salleh Ismail, which is assigned to Touchdown Technologies, Inc. the same assignee of the present application. The '568 application discloses a laterally compliant interposer as shown in FIGS. 12A and 12B. FIG. 12A illustrates a lateral interposer (1200) in an unengaged state, that is, the interposer substrate (1205) is not in a position wherein it can affect an electrical contact between the PCB (1210) and the probe contactor substrate (1215). Note that the interposer contactor (1220) does not make contact with the either the contact pad/bump (1225) on the probe contactor substrate (1215) or the contact pad/bump (1230) on the PCB (1210). In FIG. 12B, the interposer is engaged by shifting it in the direction of arrow 1235 such that the interposer contactor (1215) makes contact with both the pad/bumps of the PCB (1210) and the probe contactor substrate (1215) (see outlined region 1240). In the engaged position the PCB (1210) is in electrical connection with the probe contactor substrate (1215). The laterally compliant interposer is an improvement over the prior art because, inter alia, it relieves the stresses inherent in the vertical interposer, thus minimizing the planarity error and its attendant drawbacks. However, the interposer can introduce some lateral stress to the probe contactor substrate and the PCB, and the lateral interposer can be an intricate part to manufacture.
Thus a new design for an interposer is needed to overcome the deficiencies of the prior art.